1.3.1—Terms

Use the hierarchy in order: a principal shell is an energy level labelled by n; each shell contains sub-shells labelled s, p, d and, where supported, f; each sub-shell contains atomic orbitals. An orbital is an allowed region, not a fixed circular path.

Each orbital holds at most two electrons. Therefore s has 1 orbital and holds 2 electrons, p has 3 orbitals and holds 6, d has 5 and holds 10, and f has 7 and holds 14. For example, 2p means the p sub-shell in shell n = 2.

Higher principal shells are generally higher in energy, while sub-shell energy follows the supported order s < p < d < f with overlap at higher shells, including 4s/3d. s orbitals are spherical; p sub-shells contain three perpendicular dumbbell-shaped orbitals, pₓ, pᵧ and p_z.

Do not confuse shell, sub-shell and orbital, or treat an orbital as a track around the nucleus. The detailed filling and electron-count method belongs to the neighbouring electron-configuration objective; this card supplies the location, capacity and model boundaries.